I know a little bit perl, but not enough deeply to understand the next.
Reading perldelta 5.18 i found the next piece of code what is already disabled in 5.18. Not counting this, still want understand how it's works.
Here is the code and in the comments are what i understand
%_=(_,"Just another "); #initialize the %_ hash with key=>value _ => 'Just another'
$_="Perl hacker,\n"; #assign to the $_ variable with "Perl..."
s//_}->{_/e; # darkness. the /e - evauates the expression, but...
print
it prints:
Just another Perl hacker,
I tried, the perl -MO=Deparse and get the next
(%_) = ('_', 'Just another '); #initializing the %_ hash
$_ = "Perl hacker,\n"; # as above
s//%{'_';}/e; # substitute to the beginning of the $_ - WHAT?
print $_; # print the result
japh syntax OK
What is strange (at least for me) - running the "deparsed" code doesn't gives the original result and prints:
1/8Perl hacker,
I would be very happy:
if someone can explain the code, especially if someone could write an helper code, (with additional steps) what helps me understand how it is works - what happens.
explain, why the deparsed code not prints the original result.
What means the %{'_';} in the deparsed code?
The code actually executed by the substitution operator is probably actually something like
my $code = "do { $repl_expr }";
So when the replacement expression is _}->{_, the following is executed:
do { _}->{_ }
_ simply returns the string _ (since strict is off), so that's the same as
do { "_" }->{_}
which is the same as
"_"->{_}
What you have there is a hash element dereference, where the reference is a symbolic reference (i.e. a string rather than an actual reference). Normally forbidden by strict, here's an example of a symbolic reference at work:
%h1 = ( id => 123 );
%h2 = ( id => 456 );
print "h$_"->{id}, "\n"
for 1..2;
So that means
"_"->{_} # Run-time symbol lookup
is the same as
$_{_} # Compile-time symbol lookup
A similar trick is often used in one-liners.
perl -nle'$c += $_; END { print $c }'
can be shortened to
perl -nle'$c += $_; }{ print $c'
Because the code actually executed when -n is used is obtained from something equivalent to
my $code = "LINE: while (<>) { $program }";
%{'_';}
is a very weird way to write
%{'_'}
which is a hash dereference. Again, the reference here is a symbolic reference. It's equivalent to
%_
In scalar context, hash current returns a value that reports some information about that hash's internals (or a false value if empty). There's been a suggestion to change it to return the number of keys instead.
Related
When I print a variable, I am getting a HASH(0xd1007d0) value. I need to print the values of all the keys and values. However, I am unable to as the control does not enter the loop.
foreach my $var(keys %{$HashVariable}){
print"In the loop \n";
print"$var and $HashVariable{$var}\n";
}
But the control is not even entering the loop. I am new to perl.
I can't answer completely, because it depends entirely on what's in $HashVariable.
The easiest way to tell what's in there is:
use Data::Dumper;
print Dumper $HashVariable;
Assuming this is a hash reference - which it would be, if print $HashVariable gives HASH(0xdeadbeef) as an output.
So this should work:
#!/usr/bin/env perl
use strict;
use warnings;
my $HashVariable = { somekey => 'somevalue' };
foreach my $key ( keys %$HashVariable ) {
print $key, " => ", $HashVariable->{$key},"\n";
}
The only mistake you're making is that $HashVariable{$key} won't work - you need to dereference, because as it stands it refers to %HashVariable not $HashVariable which are two completely different things.
Otherwise - if it's not entering the loop - it may mean that keys %$HashVariable isn't returning anything. Which is why that Dumper test would be useful - is there any chance you're either not populating it correctly, or you're writing to %HashVariable instead.
E.g.:
my %HashVariable;
$HashVariable{'test'} = "foo";
There's an obvious problem here, but it wouldn't cause the behaviour that you are seeing.
You think that you have a hash reference in $HashVariable and that sounds correct given the HASH(0xd1007d0) output that you see when you print it.
But setting up a hash reference and running your code, gives slightly strange results:
my $HashVariable = {
foo => 1,
bar => 2,
baz => 3,
};
foreach my $var(keys %{$HashVariable}){
print"In the loop \n";
print"$var and $HashVariable{$var}\n";
}
The output I get is:
In the loop
baz and
In the loop
bar and
In the loop
foo and
Notice that the values aren't being printed out. That's because of the problem I mentioned above. Adding use strict to the program (which you should always do) tells us what the problem is.
Global symbol "%HashVariable" requires explicit package name (did you forget to declare "my %HashVariable"?) at hash line 14.
Execution of hash aborted due to compilation errors.
You are using $HashVariable{$var} to look up a key in your hash. That would be correct if you had a hash called %HashVariable, but you don't - you have a hash reference called $HashVariable (note the $ instead of %). To look up a key from a hash reference, you need to use a dereferencing arrow - $HashVariable->{$var}.
Fixing that, your program works as expected.
use strict;
use warnings;
my $HashVariable = {
foo => 1,
bar => 2,
baz => 3,
};
foreach my $var(keys %{$HashVariable}){
print"In the loop \n";
print"$var and $HashVariable->{$var}\n";
}
And I see:
In the loop
bar and 2
In the loop
foo and 1
In the loop
baz and 3
The only way that you could get the results you describe (the HASH(0xd1007d0) output but no iterations of the loop) is if you have a hash reference but the hash has no keys.
So (as I said in a comment) we need to see how your hash reference is created.
I have the following code snippet in Perl:
my $argsize = #args;
if ($argsize >1){
foreach my $a ($args[1..$argsize-1]) {
$a =~ s/(.*[-+*].*)/\($1\)/; # if there's a math operator, put in parens
}
}
On execution I'm getting "Use of unitialized value $. in range (or flip) , followed by Argument "" isn't numeric in array element at... both pointing to the foreach line.
Can someone help me decipher the error message (and fix the problem(s))? I have an array #args of strings. The code should loop through the second to n't elements (if any exist), and surround individual args with () if they contain a +,-, or *.
I don't think the error stems from the values in args, I think I'm screwing up the range somehow... but I'm failing when args has > 1 element. an example might be:
<"bla bla bla"> <x-1> <foo>
The long and short of it is - your foreach line is broken:
foreach my $a (#args[1..$argsize-1]) {
Works fine. It's because you're using a $ which says 'scalar value' rather than an # which says array (or list).
If you use diagnostics you get;
Use of uninitialized value $. in range (or flip) at
(W uninitialized) An undefined value was used as if it were already
defined. It was interpreted as a "" or a 0, but maybe it was a mistake.
To suppress this warning assign a defined value to your variables.
To help you figure out what was undefined, perl will try to tell you
the name of the variable (if any) that was undefined. In some cases
it cannot do this, so it also tells you what operation you used the
undefined value in. Note, however, that perl optimizes your program
and the operation displayed in the warning may not necessarily appear
literally in your program. For example, "that $foo" is usually
optimized into "that " . $foo, and the warning will refer to the
concatenation (.) operator, even though there is no . in
your program.
You can reproduce this error by:
my $x = 1..3;
Which is actually pretty much what you're doing here - you're trying to assign an array value into a scalar.
There's a load more detail in this question:
What is the Perl context with range operator?
But basically: It's treating it as a range operator, as if you were working your way through a file. You would be able to 'act on' particular lines in the file via this operator.
e.g.:
use Data::Dumper;
while (<DATA>) {
my $x = 2 .. 3;
print Dumper $x;
print if $x;
}
__DATA__
line one
another line
third line
fourth line
That range operator is testing line numbers - and because you have no line numbers (because you're not iterating a file) it errors. (But otherwise - it might work, but you'd get some really strange results ;))
But I'd suggest you're doing this quite a convoluted way, and making (potentially?) an error, in that you're starting your array at 1, not zero.
You could instead:
s/(.*[-+*].*)/\($1\)/ for #args;
Which'll have the same result.
(If you need to skip the first argument:
my ( $first_arg, #rest ) = #args;
s/(.*[-+*].*)/\($1\)/ for #rest;
But that error at runtime is the result of some of the data you're feeding in. What you've got here though:
use strict;
use warnings;
my #args = ( '<"bla bla bla">', '<x-1>', '<foo>' );
print "Before #args\n";
s/(.*[-+*].*)/\($1\)/ for #args;
print "After: #args\n";
eval is slow when done on a string: The string first has to be parsed before it can be executed.
I am looking for a way to cache the parsing, so that I can reuse the parsed string for yet another eval. The next eval will be the same code, but will not eval to the same value, so I cannot simply cache the results.
From the description I am looking for ceval from Eval::Compile.
But I cannot use Eval::Compile, as that requires a C compiler for the platform, and it is not given that the user has a C compiler.
So can I do something similar to ceval in pure Perl?
Background
GNU Parallel lets the user give Perl expressions that will be eval'ed on every argument. Currently the Perl expressions are given as strings by the user and eval'ed for every argument. The Perl expressions remain unchanged for each argument. It is therefore a waste to recompile the expression as the recompilation will not change anything.
Profiling of the code shows that the eval is one of the bottlenecks.
Example
The user enters: $_ .= "foo" and s/a/b/g
A user's scripts are stored in $usereval1 and $usereval2.
The user gives 10000 random arguments (strings) stored in #arguments.
sub replace {
my ($script, $arg) = #_;
local $_;
$_ = $arg;
# This is where I would like to cache the parsed $script.
eval $script;
return $_;
}
for my $arg (#arguments) {
# Loads of indirect code (in the order of 1000 lines) that
# call subs calling subs calling subs that eventually does:
$replaced1 = replace($usereval1, $arg);
$replaced2 = replace($usereval2, $arg);
# Yet more code that does stuff with $replaced1 $replaced2
}
You can store a subroutine ref like this:
perl -lwe 'my $x = eval(q( sub { my $foo = shift; $foo*2; } )); print $x->(12);'
This prints 24. You can reuse the code without the need to recompile it.
I have perl function I dont what does it do?
my what does min in perl?
#ARVG what does mean?
sub getArgs
{
my $argCnt=0;
my %argH;
for my $arg (#ARGV)
{
if ($arg =~ /^-/) # insert this entry and the next in the hash table
{
$argH{$ARGV[$argCnt]} = $ARGV[$argCnt+1];
}
$argCnt++;
}
return %argH;}
Code like that makes David sad...
Here's a reformatted version of the code doing the indentations correctly. That makes it so much easier to read. I can easily tell where my if and loops start and end:
sub getArgs {
my $argCnt = 0;
my %argH;
for my $arg ( #ARGV ) {
if ( $arg =~ /^-/ ) { # insert this entry and the next in the hash table
$argH{ $ARGV[$argCnt] } = $ARGV[$argCnt+1];
}
$argCnt++;
}
return %argH;
}
The #ARGV is what is passed to the program. It is an array of all the arguments passed. For example, I have a program foo.pl, and I call it like this:
foo.pl one two three four five
In this case, $ARGV is set to the list of values ("one", "two", "three", "four", "five"). The name comes from a similar variable found in the C programming language.
The author is attempting to parse these arguments. For example:
foo.pl -this that -the other
would result in:
$arg{"-this"} = "that";
$arg{"-the"} = "other";
I don't see min. Do you mean my?
This is a wee bit of a complex discussion which would normally involve package variables vs. lexically scoped variables, and how Perl stores variables. To make things easier, I'm going to give you a sort-of incorrect, but technically wrong answer: If you use the (strict) pragma, and you should, you have to declare your variables with my before they can be used. For example, here's a simple two line program that's wrong. Can you see the error?
$name = "Bob";
print "Hello $Name, how are you?\n";
Note that when I set $name to "Bob", $name is with a lowercase n. But, I used $Name (upper case N) in my print statement. As it stands, now. Perl will print out "Hello, how are you?" without a care that I've used the wrong variable name. If it's hard to spot an error like this in a two line program, imagine what it would be like in a 1000 line program.
By using strict and forcing me to declare variables with my, Perl can catch that error:
use strict;
use warnings; # Another Pragma that should always be used
my $name = "Bob";
print "Hello $Name, how are you doing\n";
Now, when I run the program, I get the following error:
Global symbol "$Name" requires explicit package name at (line # of print statement)
This means that $Name isn't defined, and Perl points to where that error is.
When you define variables like this, they are in scope with in the block where it's defined. A block could be the code contained in a set of curly braces or a while, if, or for statement. If you define a variable with my outside of these, it's defined to the end of the file.
Thus, by using my, the variables are only defined inside this subroutine. And, the $arg variable is only defined in the for loop.
One more thing:
The person who wrote this should have used the Getopt::Long module. There's a major bug in their code:
For example:
foo.pl -this that -one -two
In this case, my hash looks like this:
$args{'-this'} = "that";
$args{'-one'} = "-two";
$args{'-two'} = undef;
If I did this:
if ( defined $args{'-two'} ) {
...
}
I would not execute the if statement.
Also:
foo.pl -this=that -one -two
would also fail.
#ARGV is a special variable (refer to perldoc perlvar):
#ARGV
The array #ARGV contains the command-line arguments intended for the
script. $#ARGV is generally the number of arguments minus one, because
$ARGV[0] is the first argument, not the program's command name itself.
See $0 for the command name.
Perl documentation is also available from your command line:
perldoc -v #ARGV
I'm just starting in Perl and I'm quite enjoying it. I'm writing some basic functions, but what I really want to be able to do is to use those functions intelligently using console commands. For example, say I have a function adding two numbers. I'd want to be able to type in console "add 2, 4" and read the first word, then pass the two numbers as parameters in an "add" function. Essentially, I'm asking for help in creating some basic scripting using Perl ^^'.
I have some vague ideas about how I might do this in VB, but Perl, I have no idea where I'd start, or what functions would be useful to me. Is there something like VB.net's "Split" function where you can break down the contents of a scalar into an array? Is there a simple way to analyse one word at a time in a scalar, or iterate through a scalar until you hit a separator, for example?
I hope you can help, any suggestions are appreciated! Bear in mind, I'm no expert, I started Perl all of a few weeks ago, and I've only been doing VB.net half a year.
Thank you!
Edit: If you're not sure what to suggest and you know any simple/intuitive resources that might be of help, that would also be appreciated.
Its rather easy to make a script which dispatches to a command by name. Here is a simple example:
#!/usr/bin/env perl
use strict;
use warnings;
# take the command name off the #ARGV stack
my $command_name = shift;
# get a reference to the subroutine by name
my $command = __PACKAGE__->can($command_name) || die "Unknown command: $command_name\n";
# execute the command, using the rest of #ARGV as arguments
# and print the return with a trailing newline
print $command->(#ARGV);
print "\n";
sub add {
my ($x, $y) = #_;
return $x + $y;
}
sub subtract {
my ($x, $y) = #_;
return $x - $y;
}
This script (say its named myscript.pl) can be called like
$ ./myscript.pl add 2 3
or
$ ./myscript.pl subtract 2 3
Once you have played with that for a while, you might want to take it further and use a framework for this kind of thing. There are several available, like App::Cmd or you can take the logic shown above and modularize as you see fit.
You want to parse command line arguments. A space serves as the delimiter, so just do a ./add.pl 2 3 Something like this:
$num1=$ARGV[0];
$num2=$ARGV[1];
print $num1 + $num2;
will print 5
Here is a short implementation of a simple scripting language.
Each statement is exactly one line long, and has the following structure:
Statement = [<Var> =] <Command> [<Arg> ...]
# This is a regular grammar, so we don't need a complicated parser.
Tokens are seperated by whitespace. A command may take any number of arguments. These can either be the contents of variables $var, a string "foo", or a number (int or float).
As these are Perl scalars, there is no visible difference between strings and numbers.
Here is the preamble of the script:
#!/usr/bin/perl
use strict;
use warnings;
use 5.010;
strict and warnings are essential when learning Perl, else too much weird stuff would be possible. The use 5.010 is a minimum version, it also defines the say builtin (like a print but appends a newline).
Now we declare two global variables: The %env hash (table or dict) associates variable names with their values. %functions holds our builtin functions. The values are anonymous functions.
my %env;
my %functions = (
add => sub { $_[0] + $_[1] },
mul => sub { $_[0] * $_[1] },
say => sub { say $_[0] },
bye => sub { exit 0 },
);
Now comes our read-eval-loop (we don't print by default). The readline operator <> will read from the file specified as the first command line argument, or from STDIN if no filename is provided.
while (<>) {
next if /^\s*\#/; # jump comment lines
# parse the line. We get a destination $var, a $command, and any number of #args
my ($var, $command, #args) = parse($_);
# Execute the anonymous sub specified by $command with the #args
my $value = $functions{ $command }->(#args);
# Store the return value if a destination $var was specified
$env{ $var } = $value if defined $var;
}
That was fairly trivial. Now comes some parsing code. Perl “binds” regexes to strings with the =~ operator. Regexes may look like /foo/ or m/foo/. The /x flags allows us to include whitespace in our regex that doesn't match actual whitespace. The /g flag matches globally. This also enables the \G assertion. This is where the last successful match ended. The /c flag is important for this m//gc style parsing to consume one match at a time, and to prevent the position of the regex engine in out string to being reset.
sub parse {
my ($line) = #_; # get the $line, which is a argument
my ($var, $command, #args); # declare variables to be filled
# Test if this statement has a variable declaration
if ($line =~ m/\G\s* \$(\w+) \s*=\s* /xgc) {
$var = $1; # assign first capture if successful
}
# Parse the function of this statement.
if ($line =~ m/\G\s* (\w+) \s*/xgc) {
$command = $1;
# Test if the specified function exists in our %functions
if (not exists $functions{$command}) {
die "The command $command is not known\n";
}
} else {
die "Command required\n"; # Throw fatal exception on parse error.
}
# As long as our matches haven't consumed the whole string...
while (pos($line) < length($line)) {
# Try to match variables
if ($line =~ m/\G \$(\w+) \s*/xgc) {
die "The variable $1 does not exist\n" if not exists $env{$1};
push #args, $env{$1};
}
# Try to match strings
elsif ($line =~ m/\G "([^"]+)" \s*/xgc) {
push #args, $1;
}
# Try to match ints or floats
elsif ($line =~ m/\G (\d+ (?:\.\d+)? ) \s*/xgc) {
push #args, 0+$1;
}
# Throw error if nothing matched
else {
die "Didn't understand that line\n";
}
}
# return our -- now filled -- vars.
return $var, $command, #args;
}
Perl arrays can be handled like linked list: shift removes and returns the first element (pop does the same to the last element). push adds an element to the end, unshift to the beginning.
Out little programming language can execute simple programs like:
#!my_little_language
$a = mul 2 20
$b = add 0 2
$answer = add $a $b
say $answer
bye
If (1) our perl script is saved in my_little_language, set to be executable, and is in the system PATH, and (2) the above file in our little language saved as meaning_of_life.mll, and also set to be executable, then
$ ./meaning_of_life
should be able to run it.
Output is obviously 42. Note that our language doesn't yet have string manipulation or simple assignment to variables. Also, it would be nice to be able to call functions with the return value of other functions directly. This requires some sort of parens, or precedence mechanism. Also, the language requires better error reporting for batch processing (which it already supports).